In parallel power shift transmissions, which can in general be constructed in any desired technical form, torque is transmitted between an input element and an output element alternately via one or another of the respective parallel torque-transmitting branches present. In this, the change of torque transmission from a first torque-transmission branch to a second torque-transmission branch when a transmission ratio is engaged in each, takes place by time-overlapped disengagement of the power shift element associated with the first torque-transmission branch and engagement of the power shift element associated with the second torque-transmission branch, and therefore without any interruption of the traction force. To be able to carry out sequential shifts, i.e. shifts between transmission ratio steps with adjacent transmission ratios, as load shifts, i.e. without traction force interruption, the transmission ratio steps are preferably distributed in alternation in the sequence of their transmission ratios between intermediate elements. To achieve compact dimensions of the parallel shift transmission the transmission steps, expediently in groups of adjacent transmission ratios, are alternately in driving connection with a plurality of drive output elements or can be connected thereto. If the number of transmission ratio steps available is to be increased in accordance with the current development trend, then the number of transmission ratio steps per intermediate element and/or per drive output element or the number of parallel torque-transmission branches would have to be correspondingly increased, but disadvantageously, this would result in larger dimensions, higher weight and a more complex structure of the parallel shift transmission.
A known load-shiftable parallel shift transmission with two parallel torque-transmission branches is a double clutch transmission which, with a structure that is particularly axially compact and therefore especially well-suited for transverse mounting of the drive unit in a vehicle, comprises two coaxially arranged intermediate shafts and two countershafts arranged axis-parallel thereto. In a double clutch transmission of this type the intermediate shafts can each be connected on the input side, by means of an associated friction clutch such as a dry clutch or a wet-operating disk clutch, to a common input shaft and on the output side, by a plurality of gear steps, each of which can be shifted by means of an associated clutch such as a synchronized or unsynchronized claw clutch, in alternating association with the two countershafts. The countershafts can for example be in driving connection via in each case an output constant formed of two fixed gearwheels, such as an axle differential of a drive axle close to the transmission or a cardan shaft leading to an axle differential of a drive axle far from the transmission. To achieve an axially short structural length of the double clutch transmission, the gearwheels of the gears forming at least one pair of gear steps, which are associated with a common intermediate shaft and in each case one other of the two countershafts, can be arranged in a common gearwheel plane and can have a common fixed gearwheel arranged on the intermediate shaft concerned.
A first double clutch transmission of this type is known for example from DE 198 21 164 A1. In this double clutch transmission the odd-numbered gear steps (1.G, 3.G, 5.G) are associated with a central, first intermediate shaft and the even-numbered gear steps (2.G, 4.G, 6.G) with a second intermediate shaft made as a hollow shaft and arranged coaxially around the first intermediate shaft. The double clutch transmission has five gearwheel planes in which the gearwheels of the six gear steps are arranged. The gearwheels of the fourth gear step (4.G) and the sixth gear step (6.G) are arranged in a common gearwheel plane and have a common fixed gearwheel which is arranged in a rotationally fixed manner on the second intermediate shaft. The idler gearwheel of the fourth gear step (4.G), as also the idler gearwheels of the first, second and third gear steps (1.G, 2.G, 3.G), are arranged to rotate on a first countershaft and can be engaged by associated gear clutches, whereas the idler gearwheel of the sixth gear step (6.G), as also the idler gearwheel of the fifth gear step (5.G), are arranged to rotate on a second countershaft and can be engaged by an associated gear clutch. The two countershafts, together with the two coaxial intermediate shafts, form a compact, V-shaped shaft arrangement and are each in driving connection via a fixed drive output wheel with a common output element, such as an axle differential of a drive axle close to the transmission. The gear clutches associated with the idler gearwheels arranged on the countershafts are largely brought together in pairs in shifting packets.
Another double clutch transmission of this type is described in DE 198 60 251 C1. In this known double clutch transmission the odd-numbered gear steps (I, III, V) are associated with a central, first intermediate shaft and the even-numbered gear steps (II, IV, VI) with a second intermediate shaft made as a hollow shaft and arranged coaxially with the first intermediate shaft. As a special feature, the double clutch transmission comprises a second gear gearset of the fourth gear step (IV*), which is associated with the first intermediate shaft. In addition, the third gear step (III, III*) can optionally be associated with the first or with the second intermediate shaft by means of an idler gearwheel mounted to rotate on the first intermediate shaft near the outer end of the second intermediate shaft, which can optionally be coupled by means of two gear clutches either to the first intermediate shaft or to the second intermediate shaft.
By virtue of the additional fourth gear step (IV*) associated with the first intermediate shaft and the optional shiftability of the third gear step (III*) by the second intermediate shaft, shift operations in which one of the two gear steps (III, III*; IV, IV*) concerned can be bypassed, can be carried out in each case as power shifts, i.e. without traction force interruption. For this purpose, however, the known double clutch transmission comprises six gearwheel planes in which the gearwheels of the gear steps (I-VI) are arranged. The gearwheels of the third gear step (III, III*) and the additional fourth gear step (IV*) are arranged in a common gearwheel plane and comprise the already mentioned idler gearwheel mounted to rotate on the first intermediate shaft. The normal idler gearwheel of the additional fourth gear step (IV*), as also the idler gearwheels of the first, second and normal fourth gear steps (I, II, IV), are arranged to rotate on a first countershaft and can be engaged by an associated gear clutch, whereas the idler gearwheel of the third gear step (III, III*), as also the idler gearwheels of the fifth and sixth gear steps (V, VI), are arranged on a second countershaft and can be engaged by an associated gear clutch.
A further double clutch transmission of this type is known from DE 103 16 070 A1. In this known double clutch transmission the odd-numbered gear steps (A1, A3, A5) are associated with a central, first intermediate shaft and the even-numbered gear steps (A2, A4, A6) with a second intermediate shaft made as a hollow shaft and arranged coaxially around the first intermediate shaft. This double clutch transmission has only four gearwheel planes, in which the gearwheels of the gear steps (A1-A6) are arranged. The gearwheels of the third gear step (A3) and of the fifth gear step (A5) are arranged in a common gearwheel plane and have a common fixed gearwheel which is arranged in a rotationally fixed manner on the first intermediate shaft. Likewise, the gearwheels of the fourth gear step (A4) and of the sixth gear step (A6) are arranged in a common gearwheel plane and have a common fixed gearwheel which is arranged in a rotationally fixed manner on the second intermediate shaft. The idler gearwheels of the third and fourth gear steps (A3, A4), as also the idler gearwheels of the first and second gear steps (A1, A2), are arranged to rotate in each case on a first countershaft and can be engaged by a respective gear clutch, whereas the idler gearwheels of the fifth and sixth gear steps (A5, A6) are rotatably supported on a second countershaft and can in each case be engaged by an associated gear clutch.
If, in accordance with the current development trend, one of the double clutch transmissions described above is to be extended by two further gear steps, this would entail extending in each case the transmission structure concerned by one or two gearwheel planes. This would on the one hand entail considerable development costs and on the other hand unfavorably increase the axial structural length of the double clutch transmission, whereby in particular transverse mounting of the drive unit consisting of the drive motor and the double clutch transmission in a vehicle would at least be made much more difficult.
To shift a relatively compact and in particular axially short power shift transmission, in DE 102 32 831 A1 and in DE 102 32 835 A1 a double clutch transmission is proposed in each case, in which each part-transmission comprises a plurality of axis-parallel countershafts. In each part-transmission a first countershaft can be connected by means of a friction clutch to a common input shaft. In each case the first countershaft can be connected to a second countershaft by two gearsets with different transmission ratios, each of which, in alternating arrangement, consists of a fixed gearwheel and an idler gearwheel that can be engaged by means of an associated clutch. The second countershaft can in each case be connected to a third countershaft via three gearsets with different transmission ratios, which in alternating arrangement each consists of a fixed gearwheel and an idler gearwheel that can be engaged by an associated clutch. The third countershafts are in each case in driving connection via an output constant with a common drive output shaft. In the double clutch transmission according to DE 102 32 831 A1 at least some of the idler gearwheels are provided with synchronizing clutches that act by friction.
Thus, these double clutch transmissions known from DE 102 32 831 A1 and DE 102 32 835 A1, while being axially short, each have a total of twelve gear steps, each gear step being obtained by shifting two serially arranged gearsets of a part-transmission. However, because of the large number of countershafts, gearsets and clutches the structure of these double clutch transmissions is very complex and with a folded shaft arrangement, also demands a radially large installation space.